A simulator is a system that attempts to replicate, or simulate, a given experience by performing operations as realistically and as close as possible to the real experience for training, investigation or development purposes. A well known type of simulator is the flight simulator, which is used to train pilots. The majority of simulators require high performance display systems in order to emulate the visual environment in as realistic a manner as possible.
For some types of simulation, for example flight simulation, the visual environment generally consists in a set of objects located at great distances away from the observer. For such types of simulation, conventional display systems that produce a dynamic structured pattern of light, such as a matrix of pixel, on a surface, for example a screen, do not allow a realistic simulation of the visual environment. Such display systems allow the emulation of the visual context but the objects, i.e. the sources of light, still appear to come from short distances away. The visual context thus suggests large distances but the physiological responses of the eye reveal another reality. This is an important disadvantage since this contradictory information may alter the observer's perception or even cause illness. This problem is not present in the case of a display system using a collimated screen. Such a display system produces a collimated beam for each individual picture element, i.e. pixel, which emulates the real set of beams corresponding to a real scene consisting of distant objects.
For a realistic experience, images should extend over a wide angle of view. Thus, the observer has the impression of being immersed in the action when he or she is surrounded by the images over an important part of his viewing space.
The Head-Up Display (HUD) that equips most modern fighter aircrafts is a well known example of a collimated display. A HUD is used to project information in the nominal line of sight of the observer by means of a partially reflective or dichroic (color selective reflection) window. The HUD is a see through device since it allows the pilot to see both the outside scene and the projected information. The image projected by a HUD is a virtual image and appears to be located at far away in front of the pilot. This is accomplished with a large projection lens. The display is located close to the back focal surface of the optical system which results in the production of a virtual image located at a great distance away from the observer. The window may be either flat or curved. Conversely to the flat window, the curved window possesses an optical power and contributes to the optical power of the global optical system. The field of view of a HUD is generally small, 15-20 degrees and 30-40 degrees respectively for flat and curved window, respectively.
Collimated displays have been designed specifically for simulator purposes. There exist many types of such displays. The most simple type consists of a video display (CRT, LCD etc.) located at the focal plane of a large lens. A collimated beam with the specific propagation direction is produced for each pixel of the display. The observer is located on the other side of the lens and he or she sees the image as if it was located at an infinite distance in front of the device. Because of weight, size and cost, a conventional glass lens is generally not useable for this type of collimated display configuration. Referring to FIG. 1, such collimated display 10 generally use Fresnel lenses 12 placed between a display device 14 and the observer 1 in his or her line of sight 16. In practice, the collimating lens 12 may consist of a plurality of Fresnel lenses in order to allow an acceptable correction of the optical aberrations. The Fresnel lens virtual display approach has been used, for example, by McDonnell Douglas in the design of the Vital IV system and by Boeing.
Concave mirrors may also be used to make collimated displays. There exist at least two main optical configurations of such collimated displays, which are the on axis and the off axis configurations. Referring to FIG. 2, the on axis configuration 20 consists of a display device 22, a beam splitter 24 and a concave mirror 26. The beams of light 23 from the display device 22 is reflected on the beam splitter 24 and reflected towards the concave mirror 26. The concave mirror 26 then reflects and collimates the beams of light 27 back through the beam splitter 24 towards the observer 1. In general, the beam splitter 24 is simply a partial reflectivity flat window and has no optical power. The beam splitter 24 acts as a folding mirror for the object space part of the optical path and as a window for the rest of the path. For the object space part of the optical path, the display system is equivalent to having the display device 22 located directly in front of the concave mirror 26 at the position corresponding to the virtual image 25 formed by the beam splitter 24. On the other hand, the off axis mirror configuration 30, shown in FIG. 3, uses an off axis portion 31 of a concave mirror 32 to reflect beams of light 35 from a display device 34 towards the observer 1 and achieve the same goal of producing a distant virtual image without requiring the use of a beam splitter. The off axis configuration 30 is much more energy efficient in comparison to its on axis counterpart 20.
There exists a variant of the concave mirror configurations 20, 30 commercialized under the name Pancake Window™ by Farrand Optical Company. The Pancake Window™ optical system 40, illustrated in FIG. 4, uses a partially reflective concave mirror 42 together with a flat beam splitter 44 and different types of polarizing components 46 (quarter-wave plates 46a and linear polarizers 46b). The polarizing components 46 are used to eliminate undesirable ghost images and reflections. The polarizing components 46 have the shape of thin flat sheets having their normal axis coincident with the optical axis 41 of the optical system 40. The beam splitter 44 also has its normal coincident with the optical axis 41 of the optical system 40. The display device 48 is located behind the concave mirror 42. In the object space path 49, the beams of light 47 from the display device 48 pass through the concave mirror 42 and are reflected back toward the concave mirror 42 by the beam splitter 44. The beams of light 47 are then reflected by the concave mirror 42 and pass through the beam splitter 44. Most of the beams of light 47 with optical paths different than the nominal path 45 are eliminated by the polarizing components 46. In an alternative version, the concave mirror 42 may be replaced by a holographic element with similar optical properties. The optical system 40 is relatively compact and provides a relatively large field of view (60 degrees by 90 degrees) but it is not energy efficient. It has a transmission of only about 1%. This is an important drawback since this imposes a limitation on the image brightness.
Collimated displays have also been made using only holographic elements, as shown in FIG. 5. Such collimated display 50 consists of a holographic diffusing screen 52 with a holographic lens 54. The image to be displayed is projected by a projector 56 on the holographic diffusing screen 52. The holographic diffusing screen 52 is used to control the light divergence to ensure that each image pixel produces a cone of light which illuminates the entire surface of the holographic lens 54. The intermediary image 53 on the holographic diffusing screen 52 is located on the front focal plane of the holographic lens 54. The holographic lens 54 produces a virtual copy of the image at an infinite distance. Hence, a large collimated beam is produced for each pixel of the input image.
The above-presented collimated displays do not have large enough field of view to provide an immersive sensation. Larger field of view may be achieved by producing a mosaic of several Pancake Windows™, Fresnel lens virtual displays, holographic collimated displays or others single channel collimated displays. Very large field of view may be achieved with the mosaic approach at the expense of the complexity related to the calibration required to obtain uniform properties and the necessity to drive all of those displays simultaneously. In addition to the complexity, another drawback of the mosaic approach is the fact that it does not provide a truly continuous image since there are dead zones in between adjacent displays.
Continuous displays with a medium field of view may be made using an off axis mirror with a plurality of display devices. In comparison with the single channel off axis mirror display, the field of view increase is generally achieved for only the horizontal direction. For some of those systems, the images are produced with video projector and projected on a curved (with aspheric shape) screen which acts as a secondary image. The light diffused from the screen is then reflected and collimated by the off axis mirror. Such devices are generally used for commercial airplane flight simulators and vehicle simulators.
The realization of a wide angle display is challenging as such a device must produce a large number of collimated beams coming from a very large range of directions and, using conventional approaches as described above, involves large optical components.
In the present specification, there are described embodiments of a wide angle display system designed to overcome the above-described limitations of the conventional display systems.